Kasturi Saha

Octave-spanning frequency comb generation in a silicon nitride chip

We demonstrate a frequency comb spanning an octave via the parametric process of cascaded four-wave mixing in a monolithic, high-Q silicon nitride microring resonator. The comb is generated from a single-frequency pump laser at 1562 nm and spans 128 THz with a spacing of 226 GHz, which can be tuned slightly with the pump power. In addition, we investigate the RF amplitude noise characteristics of the parametric comb and find that the comb can operate in a low-noise state with a 30 dB reduction in noise as the pump frequency is tuned into the cavity resonance.

Modelocking and femtosecond pulse generation in chip-based frequency combs

We investigate simultaneously the temporal and optical and radio-frequency spectral properties of parametric frequency combs generated in silicon-nitride microresonators and observe that the system undergoes a transition to a mode-locked state. We demonstrate the generation of sub-200-fs pulses at a repetition rate of 99 GHz. Our calculations show that pulse generation in this system is consistent with soliton modelocking. Ultimately, such parametric devices offer the potential of producing ultra-short laser pulses from the visible to mid-infrared regime at repetition rates from GHz to THz.

Silicon-based monolithic optical frequency comb source

We demonstrate the generation of broad-bandwidth optical frequency combs from a CMOS-compatible integrated microresonator. We characterize the comb quality using a novel self-referencing method and verify that the comb line frequencies are equidistant over a bandwidth of 115 nm (14.5 THz), which is nearly an order of magnitude larger than previous measurements.

Chip-based frequency combs with sub-100 GHz repetition rates

By fabricating high-Q silicon-nitride spiral resonators, we demonstrate frequency combs spanning over 200 nm with free spectral ranges (FSRs) of 80, 40, and 20 GHz using cascaded four-wave mixing. We characterize the RF beat note for the 20 GHz FSR comb, and the measured linewidth of 3.6 MHz is consistent with thermal fluctuations in the resonator due to amplitude noise of the pump source. These combs represent an important advance towards developing a complementary metal-oxide-semiconductor (CMOS)-based system capable of linking the optical and electronic regimes.

High-Performance Silicon-Nitride-Based Multiple-Wavelength Source

We demonstrate a stable complementary metal-oxide-semiconductor-compatible on-chip multiple-wavelength source by filtering and modulating individual comb lines from a parametric optical frequency comb generated in a silicon nitride microring resonator. We show comb operation in a stable low-noise state. Bit-error rate measurements demonstrate negligible power penalty from six independent frequency comb lines when compared with a tunable diode laser baseline. Open eye diagrams confirm the fidelity of the 10 Gb/s data transmitted at the comb frequencies and the suitability of this device for use as a fully integrated silicon-based wavelength-division-multiplexing source.

Broadband parametric frequency comb generation with a 1-μm pump source.

We report the first experimental demonstration of broadband frequency comb generation from a single-frequency pump laser at 1-μm using parametric oscillation in a high-Q silicon-nitride ring resonator. The resonator dispersion is engineered to have a broad anomalous group velocity dispersion region near the pump wavelength for efficient parametric four-wave mixing. The comb spans 55 THz with a 230-GHz free spectral range. These results demonstrate the powerful advantage of dispersion engineering in chip-based devices for producing combs with a wide range of pump wavelengths.

Enhanced two-photon absorption in a hollow-core photonic-band-gap fiber

We show that two-photon absorption (TPA) in rubidium atoms can be greatly enhanced by the use of a hollow-core photonic-band-gap fiber. We investigate off-resonant, degenerate Doppler-free TPA on the 5S{sub 1/2{yields}}5D{sub 5/2} transition and observe 1% absorption of a pump beam with a total power of only 1 mW in the fiber. These results are verified by measuring the amount of emitted blue fluorescence and are consistent with the theoretical predictions which indicate that transit-time effects play an important role in determining the two-photon absorption cross section in a confined geometry.

Asynchronous single-shot characterization of high-repetition-rate ultrafast waveforms using a time-lens-based temporal magnifier

We demonstrate asynchronous, single-shot characterization of an ultrafast, high-repetition-rate pulse source using a time-lens-based temporal magnifier. We measure a 225 GHz repetition-rate pulse train from a microresonator-based frequency comb. In addition, we show that such a system can be used as a frequency compressor for real-time, high-speed RF spectral characterization.

Frequency translation via four-wave mixing Bragg scattering in Rb filled photonic bandgap fibers

We demonstrate frequency translation of a weak signal beam with 21% efficiency in Rb vapor confined to a hollow core photonic band-gap fiber via Bragg scattering by four-wave mixing using microwatt level pump beams.

Quantum interpolation for high-resolution sensing

Significance Nanoscale magnetic resonance imaging enabled by quantum sensors is a promising path toward the outstanding goal of determining the structure of single biomolecules at room temperature. We develop a technique, which we name “quantum interpolation,” to improve the frequency resolution of these quantum sensors far beyond limitations set by the experimental controlling apparatus. The method relies on quantum interference to achieve high-fidelity interpolation of the quantum dynamics between hardware-allowed time samplings, thus allowing high-resolution sensing. We demonstrate over two orders of magnitude resolution gains, and discuss applications of our work to high-resolution nanoscale magnetic resonance imaging. Recent advances in engineering and control of nanoscale quantum sensors have opened new paradigms in precision metrology. Unfortunately, hardware restrictions often limit the sensor performance. In nanoscale magnetic resonance probes, for instance, finite sampling times greatly limit the achievable sensitivity and spectral resolution. Here we introduce a technique for coherent quantum interpolation that can overcome these problems. Using a quantum sensor associated with the nitrogen vacancy center in diamond, we experimentally demonstrate that quantum interpolation can achieve spectroscopy of classical magnetic fields and individual quantum spins with orders of magnitude finer frequency resolution than conventionally possible. Not only is quantum interpolation an enabling technique to extract structural and chemical information from single biomolecules, but it can be directly applied to other quantum systems for superresolution quantum spectroscopy.